Errors were encountered while processing:
/var/cache/apt/archives/libfglrx_1%3a12-6-2_amd64.deb

A warning about xserver-xorg-video-radeon was displayed during configuration. It asked if the package fglrx should be kept, because the new xserver-xorg-video-radeon was to be installed. The advice was get rid of fglrx. I answered yes at n-curses question, and fglrx was deleted. But libfglrx... was not. My question now is: Can (or must) I remove libfglrx by hand?

You should probably just remove it. fglrx 12-6 dropped support for your card (and all RadeonHD 2000-4000's), so if you want to use fglrx, you can't use the repo version (you would have to manually install Catalyst 12-4).